Outdoor luminaires and street lighting in particular are commonly required to meet dark sky regulations and/or cutoff classifications for their light output, distribution and up-light contribution. Typical cutoff distributions, as defined by known illumination standards, are designated as having less than 25 candelas per 1,000 lamp lumens emitted at angles 90 degrees and above and less than 10 percent or 100 candelas per 1,000 lamp lumens emitted at or above angles 80 degrees from nadir. Such cutoff distributions require up-light contributions of less than 2½ percent of the luminaires lumen output. Cutoff and semi-cutoff designations for exterior luminaires and street lighting are accomplished through various known techniques including utilizing exterior hoods or prismatic combinations in order to redirect light. Further, traditional shaped acorn luminaires have typically been fabricated from full prismatic globes or textured surface globes. These various types of known globes produce a significant amount of up-light, possibly as much as 30 percent of the luminaires lumen output due to reflection on these surfaces. Up-light shields, as mentioned, are known to be added to exterior portions of the acorn luminaires in order to create cutoff type designation or distribution. However, utilizing such techniques greatly reduces the luminaire efficiency while still providing 6 percent or more of up-light. Many of these known techniques include utilization of full prismatic globes which incorporate horizontally extending prismatic surfaces in order to reflect and refract the light in the desired distribution. These full prismatic globes typically utilize prisms disposed on exterior surfaces of the reflector section and refractive prisms disposed on the exterior or interior surfaces in combination in order to redirect light through a focal point interior of the globe. Further, designs include utilization of multiple refractive zones which act in combination to selectively vary light distribution characteristics of vertical and lateral angles and intensities. Most of these prior art systems however, rely upon the utilization of these external structures such as prismatic surfaces and the like on the globe or reflector/refractor as it is typically very difficult to implement utilization of a cutoff classification acorn style luminaire utilizing only internal optics and systems. Many times these difficulties are overcome in prior art systems through the utilization of reflector or refractor systems in combination with shrouds which overly enclose the lamp and provide significant additional reflective surfaces on the globe.
These issues indicated above are even more difficult to overcome in a acorn globe style luminaire wherein cutoff classification through the use of internal optics successfully produces a cutoff distribution with up-light contributions desired to meet proper cutoff classification criteria. This is particularly the case in acorn style luminaire designs where the bottom portion of the globe is blocked by the top of the mounting pole and support base for the luminaire itself and not an open bottom such as is found in open ended reflector/refractors. Thus, for low up-light classification acorn style luminaires to meet proper cutoff criteria, light must be angled within a band extending below 90 degrees horizontal to an area directly around the lamp support base and lamp post.
Such a design requirement is met by providing in one of the present embodiments a low up-light cutoff classification acorn style luminaire having a semi-prismatic globe enclosing a space and mounted on the top of a mounting pole, the semi-prismatic globe having an open aperture at a lower end, the open aperture at the lower end having a mounting collar mounted in between the globe and the mounting pole, the mounting collar supporting a lamp within the enclosed space, the semi-prismatic globe having a plurality of substantially vertically extending prisms on an interior surface thereof and having a substantially smooth exterior, the lamp positioned within the enclosed space and substantially surrounded by a reflector, the reflector enclosing the lamp in order to shield all the direct light contributions from the lamp being emitted at angles above 90 degrees from nadir, wherein the plurality of substantially vertical prisms extend from the lower edge of the globe upward towards a substantially vertical section of the globe along a curved section of the globe.
Furthermore, one embodiment of the present invention includes a low up-light cutoff semi-prismatic acorn style luminaire having a semi-prismatic globe defining an interior space, a reflector retained within the interior space and substantially enclosing and surrounding a lamp, the arc lamp having a center point, the center point of the lamp positioned within said reflector, the reflector having a supporting bracket for supporting the reflector within the globe wherein the reflector encloses the lamp and shields all direct contributions of light emitted by the lamp at angles above 90 degrees from nadir, the globe free from prismatic structures on an exterior surface, the globe having a substantially vertical section and a curved section, wherein the curved section has a plurality of substantially vertically extending prisms interspaced on an interior wall, the plurality of prisms positioned in order to minimize the reflected light from the lamp and to redirect the reflected light by reflecting it below 90 degrees from nadir, the curved section of the globe mounted to a mounting collar for mounting of the globe onto a mounting pole, the lamp mounted within the globe and interior of said reflector by a lamp support mechanism in order to properly position the lamp within the reflector.